1. Field of the Invention
The present invention relates to a monitoring circuit for a communication device which transfers a message to another device in the form of a pulse-string signal.
2. Description of Related Art
Conventional monitoring circuits are disclosed in Japanese patent application laid-open Nos. Heisei 2-181839 and Heisei 2-184938. The monitoring circuits are called "watch-dog" timers. A device to be monitored outputs a pulse at predetermined time intervals, which are then input to the watch-dog timer. The monitored device is, for example, a microprocessor. The watch-dog timer then judges that an abnormal condition of the monitored device has occurred when the monitored device does not output a pulse signal to the watch-dog timer for more than a predetermined reference time from when it has output a previous pulse signal.
However, the conventional watch-dog timer requires that a pulse-string signal having a cycle time of more than the reference time be input and monitored for its presence in order to detect if an interval of the successive pulse signals is longer than the reference time. Therefore, circuits which are to be monitored must provide additional hardware in order to produce the continuous pulse-string signal. It follows that manufacturing costs are higher due to the additional hardware.
Moreover, the watch-dog timer can only monitor device which is directly connected with the watch-dog timer and which outputs the pulse signals more often than the reference time. When the device to be monitored is located in the middle-stage of a communication system, and an earlier-stage communication system is in an abnormal condition, the middle-stage device may not output an abnormal communication signal related to the earlier-stages of the communication system. Therefore, the conventional watch-dog timer cannot detect an abnormal situation of the previous stages of a communication system unless the monitored device outputs pulse signals which have an abnormal cycle.